A.M. Wilhelm

Emulsification by ultrasound: drop size distribution and stability.

The aim of this work is to compare the oil-in-water emulsions produced by mechanical agitation (Ultra-Turrax, 10,000 rpm, P = 170 W) or power ultrasound (ultrasound horn, 20 kHz, 130 W) using the same model system: water/kerosene/polyethoxylated (20 EO) sorbitan monostearate. The following parameters were varied: emulsification time, surfactant concentration, consumed power and volume fraction of oil. With ultrasound, the drop size (Sauter diameter, d32) is much smaller than that given by mechanical agitation under the same conditions, which makes insonated emulsions more stable. For a given drop size (d32), less surfactant is required.

Ultrasound Emulsification—An Overview

ABSTRACT Fundamentals and applications of ultrasound emulsification are reviewed. The importance of cavitation is stressed, as also is power input to the multiphase fluid. The influence of surfactants, polymeric stabilizers, temperature, pressure, and ultrasonic parameters such as frequency, residence time, acoustic intensity, and energy density are described. The effects of other physicochemical parameters such as emulsifier concentration, disperse phase volume fraction, and viscosity are discussed. Applications to both water-in-oil and oil-in-water emulsions are discussed.

Some aspects of the design of sonochemical reactors

The magnitudes of collapse pressures and temperatures as well as the number of free radicals generated at the end of cavitation events are strongly dependent on the operating parameters of the equipment namely, intensity and frequency of irradiation along with the geometrical arrangement of the transducers and the liquid phase physicochemical properties, which affect the initial size of the nuclei and the nucleation process. In the present work, the effect of these parameters on the collapse pressure generated and the maximum size of the cavity during the cavitation phenomena have been studied using the bubble dynamics equation, which considers the compressibility of the medium and a single bubble in isolation. The different liquid phase properties considered include, liquid vapor pressure, viscosity, bulk liquid temperature, surface tension and nature of dissolved gases (polytropic constant of the gas). The theoretical predictions have been also compared with the experimental results observed in the literature qualitatively and some recommendations have been made for the selection of the operating parameters so as to achieve maximum benefits. The work presented here is novel in sense that no earlier studies have considered the compressibility of the liquid medium and tried to evaluate the effect of all the operating parameters on the cavitational activity.

Characterisation of the acoustic cavitation cloud by two laser techniques

An experimental investigation of the size and volumetric concentration of acoustic cavitation bubbles is presented. The cavitation bubble cloud is generated at 20 kHz by an immersed horn in a rectangular glass vessel containing bi-distilled water. Two laser techniques, laser diffraction and phase Doppler interferometry, are implemented and compared. These two techniques are based on different measuring principles. The laser diffraction technique analyses the light pattern scattered by the bubbles along a line-of-sight of the experimental vessel (spatial average). The phase Doppler technique is based on the analysis of the light scattered from single bubbles passing through a set of interference fringes formed by the intersection of two laser beams: bubble size and velocity distributions are extracted from a great number of single-bubble events (local and temporal average) but only size distributions are discussed here. Difficulties arising in the application of the laser diffraction technique are discussed: in particular, the fact that the acoustic wave disturbs the light scattering patterns even when there are no cavitation bubbles along the measurement volume. As a consequence, a procedure has been developed to correct the raw data in order to get a significant bubble size distribution. After this data treatment has been applied the results from the two measurement techniques show good agreement. Under the emitter surface, the Sauter mean diameter D(3, 2) is approximately 10 microm by phase Doppler measurement and 7.5 microm by laser diffraction measurement at 179 W. Note that the mean measured diameter is much smaller than the resonance diameter predicted by the linear theory (about 280 microm). The influence of the acoustic power is investigated. Axial and radial profiles of mean bubble diameters and void fraction are also presented.

Oxidation of phenol in wastewater by sonoelectrochemistry

Abstract Mass transfer was investigated in ultrasonic reactors by means of the electrochemical method (feri-ferro-cyanide couple, under diffusion limitations). This method allowed for the determination of the active zones in the reactor. The oxidation of a model pollutant, phenol, has been carried out in the previously screened reactor, with ultrasound alone and with ultrasound associated with electrolysis. With a 20 kHz sonication, the electrochemical oxidation of phenol in NaCl media allows the conversion of 75% of initial phenol within 10 minutes of treatment. However a toxic intermediate, p.quinone was formed. At 500 kHz a conversion of 95% of the initial phenol was obtained within the same treatment time, and final products of degradation were acetic and chloroacrylic acids.

Destruction of phenol using sonochemical reactors: scale up aspects and comparison of novel configuration with conventional reactors

Acoustic cavitation is known to produce conditions extremely suitable for the destruction of pollutants but the industrial use is hampered by the lack of suitable scale-up/design strategies and lack of studies at pilot scale of operation. In the present work, the efficacy of acoustic cavitation for the destruction of phenol has been investigated using a novel triple frequency flow cell with a maximum capacity of 7.5 l. The results obtained have been compared with the conventional laboratory scale ultrasonic horn, which is most commonly used for the applications based on acoustic cavitation and another side entering sonochemical reactor with capacity of 400 ml again based on irradiation with single transducer at frequencies similar to the conventional horn. In the novel sonochemical reactor, the effect of frequency of irradiation on the rates of degradation has also been investigated. Some process intensification studies have also been carried out using surface cavitation in the large-scale reactor as well as the ultrasonic horn with an aim of increasing the rates of degradation.

Mapping of an ultrasonic horn: link primary and secondary effects of ultrasound

The erratic behaviour of cavitational activity exhibited in a sonochemical reactor pose a serious problem in the efficient design and scale-up; thus it becomes important to identify the active and passive zones existing in the reactor so as to enable proper placement of the reaction mixtures for achieving maximum benefits. In the present work mapping of ultrasonic horn has been carried with the help of local pressure measurement using a hydrophone and estimation of amount of liberated iodine using the Weissler reaction and a quantitative relationship has been established. The measured local pressure pulses have been used in the theoretical simulations of the bubble dynamics equations to check the type of cavitation taking place locally and also estimate the possible collapse pressure pulse in terms of maximum bubble size reached during the cavitation phenomena. Relationship has been also established between the observed iodine liberation rates and the maximum bubble size reached. The engineers can easily use these unique relationships in efficient design, as the direct quantification of the secondary effect is possible.

Modelling of free radicals production in a collapsing gas-vapour bubble

This paper deals with a model linking bubble dynamics under an acoustic pressure field and production of free radicals in the resulting collapses of this bubble. The bubble dynamics model includes interdiffusion of gas and vapour in the bubble as well as evaporation or condensation at the interface, and it assumes uniformity of the internal pressure and perfect gas law for the gas vapour mixture. At the maximum compression of the bubble, all the reactions of dissociation which can occur are assumed at thermodynamic equilibrium. The local composition (especially in free radicals) in the bubble is then calculated by an algorithm based on free energy minimization using the information concerning the maximum compression provided by the bubble dynamics model resolution. Using this model a comparison of free radicals production has been made for two different driving frequencies (20 kHz and 500 kHz), and at given bubble radius and acoustic pressure, an optimum of liquid bulk temperature has been derived for the production of free radicals very similar to the experimental one concerning oxidation reactions in aqueous phase.

Emulsification processes: on-line study by multiple light scattering measurements

The use of ultrasound in various processes of the chemical industry has been a subject of research and development for many years. As regards in emulsification, apart from formulation variables, power is the most important parameter. Efficiency of emulsification processes may be followed and evaluated by measuring particle size distribution, which mainly governs the kinetic stability of such dispersions. Unfortunately, this kind of measurement must be performed at high dilution (low volume fraction of dispersed phase). The present work is devoted to the on-line study of ultrasound emulsification by means of a newly developed apparatus based on multiple light scattering, which allows us to determine average droplet diameter and its variations directly on concentrated media. The model system was an oil (kerosene)-in-water emulsion stabilized by a polyethoxylated sorbitan monostearate.

Phenol wastewater treatment by a two-step adsorption-oxidation process on activated carbon

Abstract The interest of a two-step adsorption–oxidation process for treatment of aqueous phenolic effluents has been investigated. This process is based on the use of activated carbon as adsorbent in the first step and as oxidation catalyst in the second step, in a single bi-functional reactor. The main advantage of this process concerns the regeneration–oxidation step, for which only a small quantity of liquid has to be heated and pressurised, reducing then the heat consumption. Calculations and design were performed based on the experimental results obtained separately for the adsorption and the oxidation steps. This two-step adsorption–oxidation process appears to offer good potentialities for treating moderate flow rates of wastewater, especially when the effluent is dilute.